AC Drive Systems

Abstract

The speed of an AC motor depends on the speed of the rotating magnetic field, which is proportional to the frequency of the AC supply. Speed control is therefore possible with a power electronic controller which can generate a variable frequency. The majority of AC variable-speed drive systems use a DC/AC inverter to generate the required variable frequency from a DC source. Since the primary power source is usually the AC mains, an AC/DC converter is also required; a frequency conversion system of this kind is termed a DC link converter. It is also possible to convert power directly from the fixed AC supply frequency to a variable output frequency with a cycloconverter [1] which does not use a DC link. Converters of this kind are generally restricted to output frequencies of not more than one third of the input frequency, so they are not widely used; they will not be considered further in this book.